NGC4736 (M94) is a bright (R)Sab(r) galaxy (de Vaucouleurs 1976 - RC2), member of Canis Venatici I group (Geller & Huchra 1983). The galaxy is well studied in its optical morphology, kinematics of the optically emitting gas (van der Kruit 1974, 1976; Buta 1984, 1988) and HI distribution and kinematics (Bosma et al. 1977; Mulder & van Driel 1993).
We use in this paper Mulder & van Driel's (1993) distance to
NGC4736 (6.6 Mpc, 
. These authors give a
summary of NGC4736 basic properties.
Observations of HII regions in external galaxies are important as
tracers of high mass star formation. They provide information about
the interstellar medium from which they are formed, as well as stellar
population to which they give origin. Hodge & Kennicutt
(1983) have cataloged 54 HII regions of NGC4736. They are
mainly distributed in a ring of 3.2kpc in diameter. CCD H
fluxes
and diameters of three HII regions were measured by Kennicutt
(1988). The chemical abundances of two of them were studied by
McCall et al. (1985). Kennicutt et al. (1989)
studied the luminosity function of 78 HII regions of NGC4736, by means
of photographic photometry. Oey & Kennicutt (1993) measured
the ratios [OII]/H
and [OIII]/H of nine HII regions of NGC4736
in an abundance study of HII regions in early-type spiral galaxies.
In the present paper we study the physical conditions of NGC4736 HII
regions by means of CCD imaging in strong lines and nearby continua.
In Sect. 2 (click here) we describe the observations and data reduction
procedures. In Sect. 3 (click here) we catalogue the HII regions position,
size, absolute integrated flux in the [OII]
, H
[OIII]
, H [SII]
and
[SIII]
lines and continua intensity at 3812 Å, 4556 Å,
5276 Å, 6269 Å and 9193 Å. We derive in Sect. 4 (click here) the HII
regions physical conditions, their diameters distribution, luminosity
function, metal abundances and ionizing sources temperature. We
also discuss in Sect. 4 (click here) the spatial distribution of the nuclear
emission and the possible origin of the H P-Cygni
profile observed by Filippenko & Sargent (1985) in this
region.
NGC4736 brightness distribution was discussed by different authors.
Bosma et al. (1977) distinguish 5 morphological zones in this
galaxy (see image in Lynds 1974 and Mulder & van Driel
1993): A bulge 
; an inner spiral structure 
bounded by a bright inner ring where the majority of the
HII\
regions are located; an outer spiral structure 
,
this region probably forms an oval disk; a gap of lower surface
brightness; and a faint outer ring 
. Furthermore,
surface photometry reveals the presence of sub-structures within
Bosma's zones, such as, a 
long central bar (Kormendy
1993; Möllenhoff et al. 1995 - MMG), isophotal
twisting in the region between 
and 
(Varela et al.\
1990; Beckman et al. 1991), etc. We also discuss
in this paper the morphology of NGC4736 at different wavelengths.
The high S/N of our continua data set allows to study NGC4736
brightness and color distributions and to infer on the stellar
population radial distribution (Sect. 5 (click here)). Our conclusions
are given in Sect. 6 (click here).
Figure 1: a) NGC4736 H continuum subtracted image in logarithmic
grayscale, as indicated by the color bar. b) Identification chart of
the NGC4736 HII regions